1. Field of the Invention
This invention relates to a method for treating polyvinyl chloride suspension resins to reduce the vinyl chloride monomer content thereof.
2. Description of the Prior Art
In conventional processes for producing suspension polyvinyl chloride resins, the unreacted vinyl chloride monomer is removed from the suspension and/or resin in a number of different steps in the finishing process. A major portion of the unreacted vinyl chloride monomer may be removed by evacuating the polymerization autoclave to flash the unreacted monomer. Alternatively, the resin-monomer-water product may be transferred to a degassing vessel and vacuum applied to flash the monomer which is then condensed or otherwise recovered. Further reductions of the monomer content of the resin-water suspension can occur in vented holding or staging tanks wherein the product from the autoclave is held for some period of time prior to further processing. The vinyl chloride monomer content is significantly lowered in the final dried PVC resin product during the drying step. The PVC slurry is then separated into aqueous and dry components by conventional means such as centrifuging and/or filtering, and the wet filtrate is then dried by conventional means such as a rotary dryer or a dryer which blows heated, dehumidified air through the resin particles as they are conveyed through a closed system or as they are contained in a drying vessel. Conventional suspension polyvinyl chloride resins dried according to the foregoing conventional procedures have been found to contain, for example, from about 25 parts per million to about 4,000 parts per million vinyl chloride monomer in the final dried resin product, depending upon the nature of the particular resin produced.
Recently the vinyl chloride monomer content of polyvinyl chloride resins has become a matter of great concern to the public, the government, and the industrial producers of these resins. Governmental agencies are proposing maximum levels of vinyl chloride monomer in air surrounding polyvinyl chloride production facilities. Regulations are also proposed for limiting the emission of vinyl chloride monomer to the atmosphere. Additionally, standards for maximum levels of vinyl chloride monomer content in PVC resins that are certified for use in producing items such as film, bottles or jars that are ultimately used to package food or cosmetic products are anticipated.
In view of the foregoing, it is seen that there is a pressing need for a process for both reducing the vinyl chloride monomer emissions from suspension polyvinyl chloride plants and also to reduce the vinyl chloride monomer content of the PVC suspension resins which are produced by these plants.